The invention relates to optical switches, and more particularly, to an improved cross-point switching element.
Optical fibers provide significantly higher data rates than electronic paths. However, effective utilization of the greater bandwidth inherent in optical signal paths requires optical cross-connect switches. In a typical telecommunications environment, signals are switched between optical fibers using an electrical cross-connect switch. The optical signals are first converted to electrical signals. After the electrical signals have been switched, the signals are again converted back to optical signals that are transmitted via the optical fibers. To achieve high throughput, the electrical cross-connect switches utilize highly parallel, and highly costly, switching arrangements. However, even with such parallel architectures, the cross-connect switches remain a bottleneck.
A number of optical cross-connect switches have been proposed; however, none of these has successfully filled the need for an inexpensive, reliable, optical cross-connect switch. One class of optical cross-connect switches depends on wavelength division multiplexing (WDM) to perform the switching. However, this type of system requires the optical signals being switched to have different wavelengths. In systems where the light signals are all at the same wavelength, this type of system requires the signals to be converted to the desired wavelength, switched, and then be re-converted to the original wavelength. This conversion process complicates the system and increases the cost.
A second type of optical cross-connect switch utilizes total internal reflection (TIR) switching elements. A TIR element consists of a waveguide with a switchable boundary. Light strikes the boundary at an angle. In the first state, the boundary separates two regions having substantially different indices of refraction. In this state the light is reflected off of the boundary and thus changes direction. In the second state, the two regions separated by the boundary have the same index of refraction and the light continues in a straight line through the boundary. The magnitude of the change of direction depends on the difference in the index of refraction of the two regions. To obtain a large change in direction, the region behind the boundary must be switchable between an index of refraction equal to that of the waveguide and an index of refraction that differs markedly from that of the waveguide.
One class of prior art TIR elements that provide a large change in index of refraction operates by mechanically changing the material behind the boundary. For example, U.S. Pat. No. 5,204,921, Kanai et al. describes an optical cross-connect based on an array of crosspoints in a waveguide. A groove at each crosspoint, may be switched xe2x80x9conxe2x80x9d or xe2x80x9coff,xe2x80x9d depending upon whether the groove is filled with an index-matching oil. The index-matching oil has a refractive index close to that of the waveguides. An optical signal transmitted through a waveguide is transmitted through the crosspoint when the groove is filled with the matching oil, but the signal changes its direction at the crosspoint through total internal reflection when the groove is empty. To change the cross-point switching arrangement, grooves must be filled or emptied. In the system taught in this patent, a xe2x80x9crobotxe2x80x9d fills and empties the grooves. This type of switch is too slow for many applications of interest.
A faster version of this type of TIR element is taught in U.S. Pat. No. 5,699,462, which is hereby incorporated by reference. The TIR element disclosed in this patent utilizes thermal activation to displace liquid from a gap at the intersection of a first optical waveguide and a second optical waveguide. In this type of TIR element, a trench is cut through a waveguide. The trench is filled with an index-matching liquid. A bubble is generated at the cross-point by heating the index-matching liquid with a localized heater. The bubble must be removed from the cross-point to switch the cross-point from the reflecting to the transmitting state and thus change the direction of the output optical signal.
If the bubble contains noncondensable gases, such as air, it takes too long (minutes) to collapse when the heater is turned off. This is not acceptable for applications that require a fast cycle time. Such a gas bubble can be removed from the cross-point by applying a force to the bubble to move it to one side. However, moving the entire bubble is slow and requires substantial power. In addition, creating a new bubble to replace the bubble removed from the cross-point consumes additional power.
What is needed, therefore, is an optical cross-point switch that can be switched rapidly and with less power than prior art cross-point switches.
The invention provides an optical switch that comprises a substrate, a planar waveguide circuit, an index-matching liquid, a working fluid and a displacing device. The planar waveguide circuit is supported by the substrate. The planar waveguide circuit and the substrate collectively define a trench that includes a first trench region and a second trench region adjacent the first trench region. The second trench region has a width greater than the first trench region. The planar waveguide circuit includes a first waveguide and a second waveguide. The waveguides intersect at the first trench region and are positioned such that light traversing the first waveguide enters the second waveguide when an index-matching material is present in the first trench region, and is otherwise reflected by said the first trench region. The index-matching liquid is located in at least part of the first trench region. The working fluid is located in the second trench region. The displacing device is coupled to the second trench region, and is for displacing part of the working fluid into the first trench region to interpose the index-matching liquid between the waveguides.
The index-matching liquid may additionally be located in the second trench region and may additionally serve as the working fluid.
The planar waveguide circuit may additionally include a third waveguide having an end terminating at the trench and positioned such that light traversing the first waveguide enters the third waveguide when no index-matching medium is present in the first trench region.
The trench may additionally include a third trench region adjacent the first trench region and remote from the second trench region. The third trench region has a width greater than the first trench region. The working fluid is additionally located in the third trench region. The working fluid and the index-matching liquid partially fill the first trench region so that a bubble of a low refractive index material additionally exists in the trench region. The displacing device is additionally coupled to the third trench region. The displacing device is for displacing part of the working fluid from the second trench region to interpose the index-matching liquid between the waveguides and is additionally for displacing part of said working fluid from the third trench region to interpose the bubble between the waveguides.
The index-matching liquid may additionally be located in the second trench region and the third trench region and may additionally serve as the working fluid.
The optical switch may additionally comprise constrictions disposed along the length of the first trench region at locations corresponding to the location of a surface of the bubble when the index-matching liquid is interposed between the waveguides and when the bubble is interposed between the waveguides.
The optical switch may additionally comprise a balance channel interconnecting the second trench region and the third trench region and having a substantially greater hydraulic resistance than the first trench region.